Productivity of Ferrous Alloys Produced by Powder Bed Fusion – Laser Beam

Doctoral thesis, 2025

The thesis investigates the impact of increasing build rates on microstructure and properties, as-built surfaces and following post-AM processing in powder bed fusion – laser beam (PBF-LB) of 316L stainless steel and low-alloy steels (4130 and 4140).

The research demonstrates that increased build rates are possible when coupled with process control and appropriate post-processing. For 316L stainless steel, the study revealed that pore characteristics (distribution and orientation) can be tailored by adjusting layer thickness, scan speed and hatch distance. Extending this to low-alloy steels, optimized processing maps enabled high-density, crack-free parts at elevated build rates by managing defect formation and in-situ tempering.

To address the rough surfaces inherent to as-built components, both the electrochemical process Hirtisation® and chemical mechanical polishing (CMP) were investigated. Hirtisation® effectively reduced surface roughness through the removal of sintered powder and preferential attacks on melt pool boundaries. This microstructure-driven removal resulted in anisotropic surface patterns when the as-built surfaces exhibited anisotropy. The combination of chemical and mechanical material removal in CMP showed no influence on the developed microstructure while significantly reducing surface roughness and inducing compressive residual stresses. The mechanical interaction with the surfaces also led to the rounding of sample edges. Both surface treatments studied highlighted the need for further optimization regarding the amount of material removal required to fully eliminate subsurface defects.

Finally, the thesis established a link between pore characteristics and fatigue life of PBF-LB 316L fabricated with high build rate. Specifically, pores generated through increased hatch distances resulted in less scatter in fatigue life compared to those generated by increased scan speeds. This reduced scatter was attributed to the more similar pore distributions and pore morphologies observed in the former case. Furthermore, the application of surface treatments (Hirtisation® and CMP) was shown to double fatigue life by effectively reducing surface defects and surface roughness.

This research concludes that significant increases in PBF-LB build rates are attainable for ferrous alloys without compromising part quality, if process parameters are carefully controlled to manage microstructure and porosity, and appropriate post-processing is implemented to optimize surface integrity and fatigue performance, thereby broadening the industrial applicability of PBF-LB.